EINDHOVEN, The Netherlands -- Scientists at Philips Research have shown that it is possible to fabricate field-effect transistors that conduct both positive holes and negative electrons within a single sheet of the same plastic material, the company said Wednesday (November 26, 2003).
The ability to build circuits from complementary transistors -- n-type transistor channels, in which electrons conduct the electric current, and p-type channels, in which holes make up the majority of charge transport -- form the basis of complementary metal-oxide-semiconductor (CMOS) ICs.
CMOS technology allows the design of digital circuits, which operate with a high robustness, low power dissipation and good noise margins.
Unfortunately, until now, organic semiconductors only showed the flow of one type of charge. This was due to a high energy barrier for either electron or hole injection from the metal source and drain electrodes, which is caused by the relatively large bandgap of organic semiconductors, Philips said.
As a result CMOS-style circuits could only be made through complex materials processing to make the n-type and p-type transistors, rather than low-cost spin coating or large-area printing.
The scientists at Philips have now realized this breakthrough in two different ways. In one approach, they used a blend of p-type and n-type materials, in combination with source and drain electrodes of gold. This results in ambipolar conduction in a single layer of this blended material, while the charge injection barrier problem is solved by mixing a material with a low energy barrier for electron injection, with a material with a low barrier for hole injection.
In the second approach, ambipolar transistor operation was achieved in a single organic semiconductor material. For this purpose, organic semiconductors were chosen with a low bandgap, thus reducing the energy barrier at the source and drain electrodes for both electrons and holes.
Both n-type and p-type transistor operation was realized in a single organic semiconductor layer and using a single type of source and drain electrodes using both approaches. The Philips scientists also realized the first working ambipolar inverter circuits that showed good noise margins and high gain values.
Eduard Meijer, a senior scientist at Philips Research, received the Else Kooi award for his PhD research that led to the discovery.